Gulf of California moisture surge

Prior to the 1970s, the consensus of meteorologists was the moisture that fueled the central and southern Arizona monsoon resulted from the movement of the Bermuda High to a more south and west position, which in turn transported water vapor to the region from the Gulf of Mexico.

However, operational meteorologists in the 1970s described episodic surges of moisture that infiltrated the area that was thought to originate in the Gulf of California.

Onset is usually in early July when the winds start to shift due to intense solar heating of the Southwest United States.

During the monsoon months, the subtropical ridge moves northward due to the development of a thermal low from the intense solar radiation.

Rains from the monsoon typically start in May or June along the western slope of the Sierra Madre Occidental and move northward, reaching southern Arizona sometime in July.

Typically, during a traditional surge, a large mesoscale convective system is located off the southern tip of the Baja peninsula.

[3] Ira Brenner continued studying gulf surges in 1974, and like Hales found that they resemble a large sea breeze with warm, moist air transported northward in the lowest 10,000 feet (3.0 km) of the atmosphere.

During the field campaign, the jet was found to be a consistent feature during widely varying synoptic conditions; with it being present at Yuma 75% of the days studied.

[6] In a 2000 Monthly Weather Review article, Fuller and Stensrud show that over the 14 years studied the easterly waves consistently produce gulf surges within three days of the trough passing the tip of Baja California.

Many characteristics of surges were seen during these events such as a heat low over the Four Corners region of the United States and increased easterly flow off the Sierra Madre Occidental leading to convective downdrafts.

In August 2003, such an event occurred in Las Vegas, Nevada where some areas received over 3 inches (7.6 cm) of rain in half an hour.